Does Orlistat burn existing fat?
Not directly. Orlistat blocks some dietary fat absorption, creating a calorie deficit - it's this deficit, not the drug itself, that leads your body to use its fat stores for energy.
How does Orlistat work and is it effective?
07 September, 2020 | Jabbar Ali - MPharm
Orlistat is a medicine that can help obese people lose weight. It works by binding to the lipase enzymes that help digest and absorb the fat you eat from your diet, and inhibiting their function. According to the NHS, this prevents around a third (30-40%) of your dietary fats from being absorbed in your diet which reduces calorie intake. The unabsorbed fat will instead pass through your digestive system.
Unlike newer treatments such as Wegovy and Mounjaro, Orlistat is a tablet with no effect on appetite - it works entirely in the gut. Available in the UK since 1998, it remains a genuine option for anyone who'd rather avoid appetite suppressors and/or injections.
In this guide, we'll cover how effective Orlistat really is, how long it takes to work, how it compares to GLP-1 injections like Wegovy and Mounjaro, and how to get the best results if you decide it's right for you.
What this article covers
- What Orlistat is and how it works
- How effective Orlistat is for weight loss
- How long Orlistat takes to work and stays in your system
- Common side effects and how to take it correctly
- How to get the best results from Orlistat
- How Orlistat compares with Wegovy and Mounjaro
What is Orlistat?
Orlistat is a weight loss medication that belongs to a class of drugs called lipase inhibitors. It's available in the UK both on prescription, under the brand name Xenical at a 120mg strength, and over the counter at a lower 60mg strength under the brand name Alli. Both versions work in exactly the same way and contain the same active ingredient - the difference is dose strength and how they're accessed.
Orlistat is licensed for adults with a BMI of 30 or above, or a BMI of 28 or above alongside a weight-related health condition such as type 2 diabetes or high blood pressure. It's intended to be used alongside a reduced-calorie, lower-fat diet, rather than as a standalone solution.
How effective is Orlistat?
In clinical trials, Orlistat combined with a reduced-fat, lower-calorie diet produces an average of around 2.4kg more weight loss than diet alone. That figure comes from a 2025 review pooling 22 studies and over 5,900 patients.
Looked at another way, roughly 35% to 55% of people taking Orlistat lost at least 5% of their body weight over a year, and around one in five lost 10% or more, compared with far fewer following the same diet without medication.
Orlistat's benefits also go beyond the scales. In the four-year XENDOS trial, Orlistat taken alongside lifestyle changes reduced the risk of developing type 2 diabetes in people living with obesity, and studies have linked it to modest reductions in blood pressure.
How long does Orlistat take to work?
Orlistat starts affecting how your body processes fat almost immediately - usually within the first day or two, since it works directly in the digestive tract rather than needing to be absorbed into the bloodstream. Early signs often show up as changes in bowel movements, since the unabsorbed fat has to pass through your system.
Meaningful weight loss takes longer. Most people notice the first signs within around two weeks, though sustained progress typically takes more than two months to become clear - which is why Orlistat is reviewed after one month rather than after a few days.
Can Orlistat help me lose weight?
Orlistat can help you lose weight as part of a low fat reduced calorie diet. You should take one capsule with each main meal; breakfast, lunch and dinner. Once you have been taking Orlistat for a month you should review your progress and measure how much weight you have lost. If you haven’t lost any weight you should stop taking Orlistat and look for another weight loss solution.
Orlistat is available for people with a BMI of over 30kg/m2, or people with a BMI of over 28kg/m2 with certain risk factors.
How long does Orlistat stay in your system?
Orlistat acts directly in the gut rather than the bloodstream, so it also clears from your system quickly once you stop taking it - most people find digestion returns to normal within roughly 48 to 72 hours of their last dose.
Does Orlistat have any side effects?
The main side effects of Orlistat involve the passing of the unabsorbed dietary fat. It is important to limit dietary fat to help reduce the frequency and severity of side effects which can include loose oily stools, abdominal cramps, flatulence, oily spotting and urgency to defecate.
How to take Orlistat
Take one tablet of Orlistat three times a day with breakfast, lunch and dinner. If you miss a meal or are eating a meal with no fat, you can skip a dose. It is also recommended you take multivitamins once a day at least 2 hours before your dose of orlistat.
Orlistat is available in prescription form as the branded version Xenical and generically as Orlistat. Orlistat is cheaper than Xenical but works in exactly the same way. Prescription strength Orlistat tablets are 120mg strength. Orlistat is available without prescription in the form of Alli which has a strength of 60mg. Both treatments work in exactly the same way.
How to get the best results from Orlistat
A few habits make a real difference to both effectiveness and side effects:
- Keep fat to around 30% of calories per meal - the biggest factor in reducing digestive side effects.
- Take your dose with, or up to an hour after, a meal containing fat; skip the dose if the meal is fat-free.
- Limit alcohol, which is calorie-dense and works against your progress.
- Stay physically active - even moderate, regular activity improves results.
- Speak to your prescriber if side effects are hard to manage or you're not seeing results after a month.
How does Orlistat compare to other weight loss drugs?
Orlistat is just one option in the world of prescription weight loss treatments. Because it works through a different mechanism to GLP-1 injections, it's worth understanding how it compares to the two leading options IQ Doctor offers.
Orlistat vs Wegovy
Wegovy (semaglutide) is a once-weekly injection that works on appetite hormones, reducing hunger and helping you feel fuller for longer - a completely different mechanism to Orlistat's effect on fat absorption. In clinical trials, Wegovy has helped people lose around 15% of their starting body weight over roughly 68 weeks, compared to around 5% for most people on Orlistat over 6 to 12 months, with a smaller group reaching 10%. Wegovy generally produces stronger results, but requires a weekly injection rather than a tablet.
Orlistat vs Mounjaro
Mounjaro (tirzepatide) works on two separate hormone receptors rather than one, which is one of the reasons it has shown particularly strong results in clinical trials - some participants have lost over 20% of their starting body weight. Like Wegovy, it's a weekly injection rather than a tablet, and it works on appetite and metabolism rather than fat absorption. For most people, Mounjaro will produce more significant weight loss than Orlistat, but Orlistat remains the more accessible, non-injectable starting point for some. Our Mounjaro vs Wegovy guide compares the two leading injections in more depth.
“The honest picture with Orlistat is that the results are more modest than the GLP-1 injections, but it remains a genuine option for the right patient, particularly anyone who wants to avoid injections. It rewards a low-fat diet, and the side effects most people worry about are largely within their control.”
Omar El-Gohary
Superintendent Pharmacist and CEO of iQ Doctor
To find out if Orlistat is suitable for you, or to explore other options, visit our Weight Loss Hub for the full range of treatment guides, or start a consultation today.
Frequently Asked Questions
Can I switch from Orlistat to Wegovy or Mounjaro later?
Yes - many people start with Orlistat and move to a GLP-1 injection later if they want stronger results, or vice versa. There's usually no need to leave a long gap between treatments, but it's worth discussing the switch with your prescriber first.
Our Clinical Authors
This page has been written by a registered UK pharmacist and reviewed by our clinical team to ensure it is accurate, clear and reflects current clinical guidance.
Last medically reviewed: 07 September, 2020
Last updated: 13 October, 2026